The advantages of using composites comprising carbon fiber reinforced epoxy or other thermosetting or thermoplastic resins in advanced structures, especially for aircraft, are well known in the industry. In recent years, carbon composite materials have begun to find wide acceptance in aircraft structures. With these new materials have come new manufacturing, maintenance and life cycle management processes, combined with new machines and analysis methods to manufacture and understand these materials.
The manufacture of composite structures in production quantities has been accomplished, at least in part, through the migration towards automated manufacturing methods. Automated tape laying and fiber placement machines make high quality production parts for the Boeing™ 787 transport aircraft. Further, the use of such automated machines along with high quality steel tooling as practiced in the prior art results in high dimensional tolerance and eliminates the need for hand touch-up of the finished fuselage. Fiber placement machines are also used in the production of the fuselage of the Raytheon™ Premier. In most scenarios, this not only reduces manufacturing costs for large volumes, but also compensates for the shortage of skilled aircraft-quality composite technicians available for manual labor.
Further, autoclaves tend to be used in production programs. However, acquisition and use of autoclaves, especially large autoclaves, in production of aircraft fuselage structures is expensive.
The prior art industry norm is to build aircraft directly to production standards when the aerodynamics, propulsion system, and dynamics are of an evolutionary nature, and the market for such aircraft is sufficiently well known. In such cases the industry or military customer can justify the large investment in production tooling, production process control, and detailed analyses, while being reasonably assured that no significant changes will be required during the aircraft ground and flight testing and certification or military qualification. In cases where substantial deviation from the known aerodynamic configurations, propulsion systems, or dynamics are attempted, or the market is not secure, low cost prototypes or research aircraft are built and flight tested before committing to a production program.
In the field of composite airframes, prior art prototype construction has not followed the same methods as production aircraft construction. Composite prototype fuselage structures tend to approximate the final airframe external shape quickly and inexpensively but do not conform to the processes or quality of typical production composite fuselage structures. By contrast, prior art aluminum prototype fuselage structures are typically made using similar processes as final production aluminum fuselage structures.
More recently, the industry has used automated machines for the manufacture of wing skins of expensive military prototypes and research aircraft. Such skins are relatively flat and mostly of single curvature: shapes that lend themselves well to manufacture using tape laying machines. Such composite skins have typically been assembled with metal internal supporting structure. No known prior art prototype uses the level of automation found, for example, in the composite airframe construction of the production Boeing™ 787 aircraft.
There is an inherent conflict between the investment required to achieve high quality, accurate, repeatable, production-type parts and the lack of resources, funding, and time usually associated with prototype parts and efforts.
The substantial differences in weights, systems, and dynamics between prototype and production composite aircraft make a prototype essentially an aero-propulsion proof-of-concept. In most cases, the prototype fully conforms to the production shape, but is built with a different fuselage structure (for example that might include foam cores), whereas production aircraft would use stringers as layed up by automated machines.
Prototyping methods for composite fuselage structures have evolved to be completely different from production methods for very good reasons. First, prototyping tends to build a fuselage with manual labor, thereby reducing capital costs. Smaller pieces are favored due to limited reach of technicians, and the out-of-refrigeration time limit of many materials. Production methods on the other hand, tend to favor larger pieces, or even unitary construction, which can be cost-effectively built using automated composite lay-up machines. Second, prototyping has greater dimensional inaccuracy and large part-to-part variations, which is considered unacceptable when building production quantities, among other things because the parts are not sufficiently interchangeable. Automated machines have high dimensional accuracy, and produce interchangeable parts that require no fitting (or other hand touch up) during assembly. But the automated machines come at a high up front or capital cost that is not justified for many protoyping jobs.
In short, it is appreciated in the prior art that the use of labor in prototyping trades off against the higher cost of tooling and machinery in production, and that it makes little sense to have both high costs of labor and high costs of equipment. What is unappreciated in the prior art is that there are instances in which it may be cost-effective to make tooling with protoyping methods, but manufacturing the final fuselage structure using automated composite layup machines.
Therefore, there is a need for a schedule- and cost-affordable composite prototyping process which provides production quality or production conforming airframes.